Technical Field
Technical Background
The present invention relates to an antenna apparatus. In particular, the present invention relates to an antenna apparatus that has a patch antenna.
Various types of antenna apparatuses are used in moving objects, such as vehicles and aircrafts. Among these antenna apparatuses, a patch antenna, which is formed on a dielectric substrate, is used, for example, in a radar that monitors the periphery of such a moving object. The patch antenna is typically configured such that a patch radiating element (a patch-shaped conductor) is formed on the dielectric substrate. In addition, a conductor portion is typically formed on a surface (referred to, hereafter, as a “substrate back surface”) of the dielectric substrate opposite the surface on which the patch radiating element is formed (referred to, hereafter, as a “substrate front surface”). The conductor portion functions as a ground plate. Furthermore, a conductor portion may also be formed on the substrate front surface, separately from the patch radiating element. This conductor portion is widely formed to reach the substrate end portions.
In the patch antenna configured as described above, when the patch antenna operates, a current (surface current) flows to the ground plate surface. The surface current is attributed to an electrical field formed between the patch radiating element and the ground plate. The surface current is transmitted to the substrate end portions and results in emission (radiation) from the substrate end portions. When the conductor portion is formed on the substrate front surface, the surface current also flows to the conductor portion, thereby causing the emission from the substrate end portions. The emission from the substrate end portions caused by the surface current is unnecessary emission that affects the performance of the patch antenna. In other words, the emission from the end portions causes disturbance in the directivity of the patch antenna.
Therefore, PTL 1 discloses a technique for suppressing the surface current flowing to the ground plate. Specifically, a plurality of conductive patches are formed in the overall periphery of the patch radiating element, on the substrate front surface of the dielectric substrate. Each conductive patch is electrically connected to the ground plate on the substrate back surface by a conductive via. This configuration composed of the conductive patch and the conductive via has a band gap (electromagnetic band gap) that inhibits the transmission of surface current through the ground plate at a certain frequency. This configuration composed of the conductive patch and the conductive via is referred to, hereafter, as an “EBG”.
A plurality of EBGs are provided in this way on the overall surface surrounding the patch radiating element. As a result, the transmission of surface current to the substrate end portions is suppressed, and disturbance in the directivity of the patch antenna is thereby suppressed.